Method for determining tack

ABSTRACT

In a method and apparatus for testing and determining the tack properties of rubbery or polymeric compositions test specimens of polymer stock are each supported on a perforated plate or grid so that the specimen completely covers the perforated area. A differential air pressure is applied across the specimen plate contact area thereby forcing the specimen against the plate and maintaining the former in position. Two test specimens of polymer stock, each thus mounted, are then pressed together under a constant static force for a predetermined time interval. Immediately thereafter a second constant static force in a counter direction is released which tends to separate the specimens. The time required for separation is measured and is an indication of the tack of the specimens.

United States Patent [19 Day [45] June 26, 1973 METHOD FOR DETERMININGTACK [57] ABSTRACT Inventor; Donald y, Matawafl. J- In a method andapparatus for testing and determining [73] Assignee: Ashland Oil Inc.,Houston, Tex. h tack pmpefties of rubbery or polymeric composi' tionstest specimens of polymer stock are each sup- Filed! P 1971 ported on aperforated plate or grid so that the speci- [211 App]. NW 139,161 mencompletely covers the perforated area. A differential air pressure isapplied across the specimen plate contact area thereby forcing thespecimen against-the [52] US. Cl- 73/150 A, 73/58, 269/21 plate andmaintaining the fonne in position Two test [51] Ill!- Cl. GOIII 19/04specimens of polymer stock, each thus mounted, are Field of Search 150150 then pressed together under a constant static force for 269/21 apredetermined time interval. Immediately thereafter a second constantstatic force in a counter direction is References Cited released whichtends to separate the specimens. The

UNITED STATES PATENTS time required for separation is measured and is anindi- 3,368,398 2/1968' Skewis 73/150 the tack the specimens 3,129,586v4/l964 vAllen et al. 3,336,797 8/1967 Raffalovichi 73/150 A 4 Claims 4Draw F'gum Primary Examiner-Richard C. Queisser AssistantExaminer-Daniel M. Yasich Attorney-Van D. Harrison, Jr., William Merer,Harold M. Dixon and Walter H. Schneider PAIENIEDmzs ms SIEEIZUFZ oo o I.2 0000 flo/ro/a 7: Jay

- INVENTOR. 7%//?7M% ATTORNEY METHOD FOR DETERMINING TACK NATURE OF THEINVENTION This invention relates to the art of measuring tack and inparticular to a new and improved process of an apparatus for measuringthe tacky adhesion properties of rubbery or polymeric compositions.

PRIOR ART In the rubber industry, an important consideration in thechoice of rubber to be used in manufacturing various products is theproperty generally referred to as tack. Tack is the peculiarcharacteristic of rubber which causes two fresh rubber surfaces toadhere or coalesce and is of great practical importance. It is by use oftack that composite rubber articles such as tires are .built. Thefreshened surfaces of the component parts are merely stuck together androlled into place. The different elements such as plies and tread splicemust hold during later handling. This can be a very severe demand andthus accurate determinations of tack are important. Many attempts .havebeen made to provide a quantitative measurement of this property. Anumber of devices have been developed for testing the tack of rubberstock in the tire industry and for adhesives. Two techniques formeasuring tack are generally used. In

and/or stickiness of rubber and other materials. Still another object ofthe invention is to provide a method and apparatus for determining tackwherein test specimens 1 may be rapidly inserted and removed from thetest apparatus. Still another object of the invention is to provide amethod and apparatus whereby the specimen tack determinations are heldrigidly in place without the bowing effect noted above. Still anotherobject of the invention is to provide method and apparatus ofdetermining tack wherein results of increased accuracy are obtained.These and other objects of the invention will be apparent to thoseskilled in the art from the de scription of the invention which follows.

SUMMARY OF THE INVENTION used are maintained in position by placing eachspeciboth techniques, the test surfaces are brought together with apredetermined constant force for a standard period of time and are thenseparated. The method used to separate. the surfaces, distinguishes thetechniques. In one the surfaces are separated at a constant rate ofspeed and the force required to effect the constant rate of separationis determined. In the second technique,

a predetermined constant force of separation is used and the timerequired to separate the test specimens is measured. This lattertechnique is utilized in a tackmeter and method described in-U.S. Pat.No. 3,368,398.

Another determination often desired is a measurement of tack between onespecimen'surface which has tack and a substrate that has no tack of itsown. An exfaces of the samples are then brought in face to face contactand force is subsequently applied to separate the two surfaces. It hasbeen noted that when the pulling force is applied to the two surfaces,thusreenforced and positioned, there is a bowing effect.

The central section of each specimen tends to pull away from the holderthus imparting a convex curvature to the specimen surface. This bowingis affected by the elasticity of the rubber and introduces considerableerror in the tack determination. In determining the tack of adhesivematerials coated onto thin flexible materials such as paper, it again isdifficult to mount specimen strips sothat they will be held rigidly andnot tear during testing.

OBJECTS OF THE INVENTION It is an object of this invention to provide animproved method and apparatus for determining the tack men against arigid-perforated plate and applying a differential pressure across thecontact area of the specimen and plate. The pressure differential issuch that the pressure on the exterior side of the specimen is greaterthan the pressure on the underside of the rigid perforated plate. Whilesuch pressure continues to be applied, the surfaces of two specimens arebrought together by a force perpendicular to the area of contact for apredetermined time interval and then a force tending to separate the twosurfaces is applied. Either the time required to effect separation at aconstant separation force or the force required to effect separation atconstant rate is measured.

BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings:

FIG. 1 is a frontal elevation view of the tack measuringapparatus ofthis invention constructed in accordance with the principles describedherein;

FIG. 2 is a partial sectional view of the specimen holding portion ofthe apparatustaken along the line 2 2 in FIG. 1; I

: FIG. 3 is a representation of the bottom surface of the upper, freemoving specimen holder depicted in FIG. 2; and

FIG. 4 is a cross-section of preferred shape of specimen utilized in theinvention and to FIG. 1 specifically.

PREFERRED EMBODIMENTS Referring now to the drawings in greater detail,it will be seen that the tackometer 2 according to the present sameplane. Flexible cord 22 rides in pulleys 14 and 16 and is connected withweight holder 24 supporting weight 26. Weight holder 24 and weight26 maybe of any desired design. Here the holder 24 is a rigid metal disk 28with a vertically extending metal rod 30 attached to the center thereof.Weight 26, as depicted here, is essentially circular in cross-sectionwith a longitudinal radial slot 32 so that weight 26 can be easilyremoved from or supported by disk 28.

The other end of cord 22 is' attached to metal rod 34 which extendsthrough a slot in support bracket 36 which is rigidly attached to post8.

The slot in bracket 36 is sufficient to permit rod 34 to move freelywith as little friction as possible. The

function of the slot in bracket 36 is to maintain the movement of rod 34in a vertical direction.

Rod 34 is rigidly attached at its lower end to metal plate 38. Asdepicted in the drawing plate 38 is metal and is circular in horizontalcross-section although this is not a necessary configuration. Rigidlyattached to the underside of plate 38 are two rigid suspension posts 40and 42. Free-moving specimen holder 44 is supported between posts 40 and42 by pivot pins 46and 48. Specimen holder 44 is a hollow box-likestructure made of sheet metal or other rigid material and has a flatbottom 50 with two parallel rows of perforations 52 therein. As shown inFIG. 3 the row of perforations does not extend the full length of theflat bottom but is limited to the central section. The two rows ofperforations are spaced apart lengthwise a width greater than the widthof the trunk portion of the test specimens (when viewed in cross-sectionas in FIG. 4). A vacuum port 54 attached to the wall of the holderenables a vacuum source (not shown) to be connected to the holder 44.

Stationary specimen holder 56 in its simpliest form is a box-likestructure similar to holder 44 and is rigidly contained withinbase-plate 4. Holder 56 has a flat top 58 containing two longitudinalrows of holes therein as shown in FIG. 3 and has a vacuum port 60 in onewall of the holder, preferably the bottom so that a vacuum source (notshown) can be connected to the interior of holder 56. In operating theapparatus the longitudinal axis of holder 44 isalways at right angles tothe longitudinal axis of holder 56 and the two test strips are alwaysbrought into contact so that their longitudinal axes are at rightangles.

At the beginning of each test run ordinarily it is desired to bring thetwo specimen surfaces together under a predetermined contact force for apredetermined time interval. And subsequently it is desired to exert aseparating force on the two test surfaces. Accordingly, it is necessarythat the difference between the weight of holder 24 including supportdisk 28 (but not weight 26) and the combined weights of rod 34, metalplate 38, support posts 40 and 42, vacuum attachment at 54 I and holder44 be sufficient to provide the desired initial contact force.Correspondingly, it is necessary that the weight 26 be of sufficientmass that when it is placed on support disk 28 the net force exerted onthe test strip surfaces is the desired separating'force.

Preferably the specimens of rubber 63 to be tested for tack are T-shapedin cross-section as shown in FIG. 4 and are in the form of a longribbon-like lengths of twelve inches or so. The specimens are preparedby pressing the rubber or polymer material into a plaque mold having theconfiguration capable of imparting the desired T-shaped cross-section.For example, the mold may be designed to give a finished plaque having araised portion in cross-section (the trunk of the T) on each specimenthree-eighths or one-half of an inch wide running the length of the 12inch by 12 inch plaque, the raised portions being spaced approximately 2inches apart center to center. If a 12 inch by 12 inch plaque is cast,six test strips can be cut from each plaque, each strip having its ownmolded, raised portion which becomes the actual test surface. Two teststrips are required for the testing procedure. Several test runs aremade with one set of test strips each test being made on a fresh contactarea.

Assuming that the polymer or rubber test strips 63 and 63' with theirrespective surfaces 62 and 64 have been prepared, a section of onepolymer or rubber specimen strip is placed so that the flat side of thestrip (the top of the crossbar of the T) is in contact with theperforated surface of one specimen holder and similarly a section ofpolymer or rubber specimen strip is placed in contact with theperforated surface of the other specimen holder. A vacuum is applied tothe interior of each holder. Atmospheric pressure exerted against eachspecimen strip presses each strip against the perforated surface of thespecimen holder and maintains it in position. By spacing the two rows ofperforations in each holder wider than the raised portion of thespecimen strip any extrusion of specimen into the perforations will notaffect the test surface itself. While the vacuum is maintained, the twospecimen holders are then brought together as shown in FIG. 1 so thatthe test strips 63 and 63' cross at right angles and the surfaces 62 and64 are brought together under a force equal to the weight of rod 34,plate 38, posts 40 and 42, vacuum attachment 54 and holder 44 less theweight of'holder 24. Weight 26 is not in place at this time. After thetwo specimen surfaces have been in contact for a desired length of time,the weight 26 is added to the holder 24 thereby exerting a separatingforce upon surfaces 62 and 64 causing the two surfaces to separate. Theperiod of time required for the two surfaces to separate is measuredeither manually with a stop watch or electronically.

Another method of applying the contact and separating forces to the testsurfaces 62 and 64 may be followed. In this method, weight 26 is notremoved from holder 28 at the time the two specimen surfacesare to bebrought together under a predetermined force. Weight 26 is left in placeand an additional weight is placed on plate 38 sufficient to generatethe desired contact force between surfaces 62 and 64. When it is desiredto apply a separating force, this weight is merely removed from plate38.

At the conclusion of a test, the vacuum in each holder is released andits strip is moved lengthwise along the holder surface until two freshopposing surfaces are exposed. The test procedure is repeated each timeusing a fresh rubber or adhesive surface.

The apparatus can be easily adapted to testing the stickiness of amaterial, i.e., itscapacity to adhere to any type of surface such as asmooth metal surface. If this measurement is desired the unlikesubstrate such as the metal specimen can be substituted for one of therubber or adhesive specimen samples and rigidly attached to one of thespecimen holders. Other adaptations of the apparatus will be obvious tothose skilled in the art.

From the foregoing disclosure, it will be apparent that the method andapparatus of this invention readily overcomes several problems in thepresent art of measuring tack. The necessity for re-enforcing the testspecimens or complicated clamping equipment to prevent bowing has beeneliminated. By eliminating bowing, the accuracy of measurement has beenimproved. Rapid testing is possible because test specimens are easilyinserted and removed from the two holders. The preparation of testspecimens is expedited through the ability to cast a number of specimensat one time.

What is claimed is:

l. A process for measuring the tack of rubbery or polymeric compositionsor the stickiness of a rubbery or polymeric composition comprising: I

a. providing a pair of supports, each support having substantiallyhorizontal, rigid surfaces, each surface having apertures therein;

b. positioning a strip of a specimen of said composition in asubstantially horizontal position upon each of said rigidhorizontalsurfaces so that all apertures in each surface are covered by saidspecimens;

c. maintaining a differential air pressure across each said specimen andsupporting rigid surface in a direction whereby each of said specimensis forced against its respective supporting rigid surface;

d. initially bringing the exposed, substantially horizontal surface ofone specimen transversely into face-to-face contact with the exposedsurface of said second specimen under a constant substantiallyvertically applied force while applying said each rubber or polymericspecimen into a longitudinal differential air pressure across each saidspecimen for maintaining said rigid horizontal surfaces andcorresponding specimens in said substantially horistrip having a T-shapein cross-section.

3. The process of claim 1 wherein said differential air pressure acrosseach specimen comprises a vacuum applied on the side of said rigidsurface opposite the side in contact with said specimen.

4. The process of claim 3 wherein said specimen comprises a longitudinalstrip having a T-shape in cross-section.

1. A process for measuring the tack of rubbery or polymeric compositionsor the stickiness of a rubbery or polymeric composition comprising: a.providing a pair of supports, each support having substantiallyhorizontal, rigid surfaces, each surface having apertures therein; b.positioning a strip of a specimen of said composition in a substantiallyhorizontal position upon each of said rigid horizontal surfaces so thatall apertures in each surface are covered by said specimens; c.maintaining a differential air pressure across each said specimen andsupporting rigid surface in a direction whereby each of said specimensis forced against its respective supporting rigid surface; d. initiallybringing the exposed, substantially horizontal surface of one specimentransversely into face-to-face contact with the exposed surface of saidsecond specimen under a constant substantially vertically applied forcewhile applying said differential air pressure across each saId specimenfor maintaining said rigid horizontal surfaces and correspondingspecimens in said substantially horizontal position; e. maintaining saidface-to-face contact of said specimens for a predetermined timeinterval; f. subsequently applying a second vertical force to saidhorizontal specimens and supporting surfaces in a direction counter tosaid initial vertical force, said second vertical force being greaterthan said initial vertical force thereby tending to separate thesurfaces of said specimens in face-toface contact; and g. measuring thetime or force required to effect separation of the two contactingsurfaces.
 2. The process of claim 1 and the step of molding each rubberor polymeric specimen into a longitudinal strip having a T-shape incross-section.
 3. The process of claim 1 wherein said differential airpressure across each specimen comprises a vacuum applied on the side ofsaid rigid surface opposite the side in contact with said specimen. 4.The process of claim 3 wherein said specimen comprises a longitudinalstrip having a T-shape in cross-section.